The use of robot arms for positioning and placing objects is well known. Generally, the arms have Z, r and motion in a conventional cylindrical coordinate system. The r or straight line movement of the end effector or mechanical hand at the end of the arm has been accomplished in a number of manners. For example, telescoping arms have been utilized for this purpose.
United Kingdom Patent Application GB 2193482A, published Feb. 10, 1988 and claiming priority from U.S. Pat. application Ser. No. 856,749 filed Aug. 5, 1986 discloses a wafer handling arm which includes two unequal length links with the distal end of one link being pivotally attached to the proximal end of the other link, with the hand being integral with the distal end of the distal link and which utilizes a belt drive which is fixed to a cam to attain nearly straight line motion.
It is also known to utilize an isosceles triangle type linkage wherein two equal length links are pivoted together and a mechanical hand is pivoted to the distal end of the distal link. Pulleys and belts are utilized in such a manner that the angle between the two links changes at twice the rate as do the angles that each of the links makes with a line connecting the points about which their other ends are pivoted. This linkage provides drive directly from a motor shaft to the proximal end portion of the proximal link. A belt about a stationary pulley coaxial with the motor shaft passes about a pulley at the point of pivoting of the two links to one another. Another pulley and belt arrangement provides pivoting of another pulley where the second link is pivotally connected to the mechanical hand.
In another apparatus a pair of isoceles triangle type linkages face one another and the mechanical hand is pivotally attached to the distal ends of both of the distal links. The proximal ends of each of the proximal links is driven in an opposite direction of rotation by a single rotating motor shaft, generally through use of appropriate gearing. What results is a frogs leg type of motion with each isoceles triangle type linkage serving as means for controlling the other such linkage in such a manner that the angles between the two links of each of the isosceles triangle linkages changes at twice the rate as do the angles that each of the links makes with a line connecting the points about which their other ends are pivoted.
In copending application Ser. No. 07/299,754 an arm structure is disclosed comprising first and second longitudinally extending links each having proximal and distal end portions, the second longitudinally extending link being twice the effective length of the first link and the proximal end portion of the second link being pivotally mounted to the distal end portion of the first link about a first pivot axis. A third longitudinally extending link is equal in effective length to the first link, the third link having proximal and distal end portions. The proximal end portion of the third link is rotatably mounted about a third pivot axis to the distal end portion of the second link. An end effector is pivotally mounted to the distal end portion of the third link for rotation about a fourth pivot axis. Means is provided for coordinatedly rotating the first link, the second link, the third link and the end effector at a rotation ratio of the first axis to the second axis to the third axis to the fourth axis of 1:2:2:1.
There is a problem which is common to all of the above described radial positioning arms. This problem is that the arms must sit idly by while a workpiece is being worked upon. For example, a semiconductor wafer is picked up from a loading cassette by an end effector located at the end of the arm. The wafer is moved to a processing station and deposited. The arm moves away and sits idly by until processing at the station is completed. Once the process is completed the single end effector must move into the processing chamber, pick up the processed wafer and retract, rotate to the receiving cassette, place the processed wafer in the receiving cassette, rotate to the loading cassette, move in and pick up another wafer, retract, rotate back to the processing chamber, place the wafer and retract again to wait for the process to be finished. This is a total of eleven movements, and the time these movements take limits the throughput, i.e., the number of eleven pieces which can be processed in a given time.
The present invention is directed to overcoming one or more of the problems as set forth above.